Stimulation of mitosis in thymocytes by concanavalin-A is accompanied by enhanced glucose transport. Inhibition of thymocyte growth by glucocorticoids is accompanied by reduced glucose transport. Goals of this project are (a) To determine the significance of these glucose-transport-modulations with respect to the anabolic responses and (b) To determine what we can of the mechanisms by which the transport modulations are brought about. Our focus to this point has been on (b). We have found rat thymocytes to consist of two distinct cell types. About 1/3 of the cell volume consists of "active" cells, which equilibrate with methylglucose rapidly (t 1/2 approximately or equal to 1 min). The other 2/3 consists of "quiescent" cells which equilibrate slowly (t 1/2 approximately or equal to 30 min). Several agents enhance transport of quiescent cells (x 30) and leave that of active cells unaffected. Enhanced Ca ions influx is an early event mediating stimulation by Con-A or A-23187 but not mediating stimulation by phenazine methosulfate or arsenate. Kinetic analysis of methyglucose transport in active cells revealed the relative mobilities of various carrier forms and dissociation constants on either side of the membrane. A similar analysis in quiescent cells appears to reveal masking by a reaction which stabilizes the empty carrier on both sides of membrane against both translocation and loading.